Effectiveness of universal adhesives on primary tooth enamel: a literature review

1. Introduction

One of the main challenges faced by pediatric dentists is achieving reliable adhesion, as microstructural and chemical differences exist between primary and permanent teeth [1, 2]. Therefore, understanding the characteristics of primary teeth is essential to elucidate the mechanisms of adhesion to the different dental substrates, namely dentin and enamel [3].

The enamel of primary teeth is thinner, has a lower mineral content, and presents a higher prism density [4]. In addition, it exhibits an aprismatic enamel layer with an approximate thickness ranging from 16 to 45 μm, which is considerably greater than that of permanent enamel, where it measures approximately 5 μm. This feature is particularly relevant, as it may hinder adhesive procedures in primary teeth [5].

Adhesive systems can be classified according to their etching strategy into two major groups: etch-and-rinse and self-etch systems [6]. The latter, also referred to as universal adhesives, multipurpose adhesives, or single-step adhesives [7, 8], differ from other adhesive systems mainly in that they contain an acidic primer and resin in a single bottle [9]. This provides the advantage of minimizing the handling of various materials, reducing chair time, and achieving shorter treatment time in pediatric patients. Furthermore, they can be used with different adhesive strategies and on a variety of substrates, providing high bond strength [10, 11].

Universal adhesives are self-etch systems to which functional phosphate and/or carboxylate monomers—such as 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate)—have been added. This hydrophobic molecule enhances chemical bonding [12, 13]. These adhesives are capable of bonding to calcium in hydroxyapatite without causing extensive demineralization of enamel and dentin [14].

Based on their degree of acidity, these adhesives can be classified into four groups: ultra-mild (pH ≥2.5), mild (pH ≈ 2), moderate (pH 1–2), and strong (pH <1) [15]. This classification is crucial, as studies indicate that the effectiveness of these adhesives is directly related to their pH. Adhesive acidity allows penetration into the smear layer without neutralizing the functional monomer [16]. In dentin, mild pH adhesives demonstrate greater bond stability with resin, whereas in enamel, selective etching appears to enhance adhesion [17]. However, these findings have been widely reported for permanent teeth; in primary teeth, the available evidence remains contradictory [18]. Some studies report improved performance of universal adhesives on primary enamel when selective etching is applied [5, 19], while others indicate no significant differences [1]. This variability provides clinicians with multiple options when selecting the most appropriate application strategy, balancing clinical judgment with theoretical considerations [20].

Based on the foregoing, further compilation and analysis of up-to-date evidence are required to determine the effectiveness of universal adhesives on the enamel of primary teeth.

2. Material and methods

Two independent reviewers identified a total of 1137 articles and selected titles for abstract screening. After this initial process, 41 potentially relevant articles were obtained for full-text assessment; prior to this, 14 duplicates were removed, resulting in 27 articles that were read in their entirety. Subsequently, exclusion criteria were applied, and a total of 14 articles were excluded as follows: 1 article in Chinese language, 2 articles with no full-text available, 2 articles that did not use any universal adhesive, 2 articles that not used resin composite as restorative material (compomer), and 7 articles that did not follow manufacturer’s instructions and had inadequate methodological quality. This resulted in a final inclusion of 13 articles. Any discrepancies were resolved through discussion between the reviewers, leading to a final consensus.

3. Results

A total of 13 studies that met the eligibility criteria were included (Fig. 1), 10 in vitro studies, 2 clinical trials, and 1 study with a mixed design that simultaneously conducted a clinical trial and an in vitro investigation (Xu et al. [21], 2023).

Fig. 1.Flow chart of the literature retrieval method of this literature review.

4. Discussion

The search for a practical adhesive system that minimizes errors during restorative procedures has led to the adoption and recommended use of universal adhesives [29]. They have been extensively evaluated in the context of permanent teeth [30] and even primary dentin [31]. However, there remains a lack of consensus regarding their application to primary enamel [6]. Thus, this literature review aims to describe the effectiveness of universal adhesives on primary tooth enamel based on evidence from in vitro and clinical studies.

One of the main limitations encountered in conducting this study was the limited number of investigations addressing this topic. Furthermore, the few studies identified, whether clinical or in vitro, were carried out under diverse conditions, which hindered the critical interpretation of the information and the adequate explanation of the findings. In this regard, particularly in clinical trials, it is necessary to standardize procedures and data collection instruments.

The analysis of the in vitro studies included in this review shows a direct relationship between selective enamel etching with phosphoric acid and the increased bond strength of universal adhesives. This relationship has been demonstrated through various testing methodologies, with the micro-shear bond strength (μSBS) [32] test emerging as the most used and recommended as it provides more accurate results. Thus, after comparing Clearfil SE Bond with Scotchbond Universal, Antoniazzi et al. [4] concluded that Scotchbond Universal demonstrated better results when applied to sound enamel in etch-and-rinse mode. They also observed that, irrespective of the etching mode, the performance of the two adhesives was affected when applied to demineralized enamel. For this reason, it is important to highlight that, in addition to the histological differences between permanent enamel and primary enamel, there are other differences between sound and demineralized primary enamel. The latter exhibits a reduction in the individual crystal content, resulting in enlargement of the intercrystalline spaces and increased tissue porosity [33]. Thus, complete removal of demineralized enamel is recommended for a predictable adhesive result, particularly given that selective removal techniques are widely used in pediatric dentistry, which may leave undetectable demineralized enamel, on which a universal adhesive performs differently [34].

Recent studies by Hamdy et al. [5] and Nozari et al. [23] evaluated the bond strength of different brands of universal adhesives applying different adhesive strategies and found that previous selective etching is key to achieving adequate adhesion to primary enamel, irrespective of the pH level of the adhesive.

Conversely, a study on permanent teeth conducted by T Suzuki et al. [35] indicated that the application of the All Bond Universal self-etching strategy (pH = 3.2) resulted in inferior adhesive values compared to more acidic adhesives such as Scotchbond Universal (pH = 2.7) and Prime&Bond Elect Universal (pH = 2.5). This suggests that manufacturers use different components (monomers such as 10 MDP, solvents such as alcohol, acetone, or water, etc.) [14] and results in variations in the acidity of the adhesive, which in turn affects their performance in self-etching applications [17]. This finding aligns with those of this review. Tichy et al. [24] compared in their study two universal adhesives with different pH levels: G-Premio Bond (pH = 1.5) and Clearfil Universal Bond Quick (pH = 2.3), and reported greater enamel surface roughness with the use of the more acidic adhesive.

Despite the encouraging results associated with universal adhesives in self-etch mode [25], these adhesives are not yet capable of producing the enamel etching patterns achieved with phosphoric acid. These patterns are deemed necessary for acceptable bond strength values in primary enamel [36], since adhesion to this tissue primarily relies on the micromechanical interlocking between the adhesive and the microporosities of the enamel [3].

The findings from the microleakage and marginal integrity tests [13, 21, 27], concluded that acid etching improved the sealing quality of the enamel-resin interface when the universal adhesive was compared with self-etch and etch-and-rinse adhesive systems, such as Clearfil SE Bond or Adper Single Bond 2, respectively. These results further support the need to use phosphoric acid prior to the application of the universal adhesive.

Various systematic reviews and meta-analyses analyzing only in vitro studies reveal contradictions with our findings. For instance, Fröhlich et al. [6] concluded that there remains uncertainty whether selective acid etching of primary enamel is necessary when universal adhesive systems are used. Similarly, da Silva et al. [3] indicated that a mild 10-MDP-based universal adhesive system can serve as an alternative to the traditional etch-and-rinse or self-etch systems for restorations of primary teeth. These affirmations should be interpreted carefully, as the scientific evidence available until 2021, when both studies were published, was limited and only 1–2 studies on primary enamel were analyzed in both cases.

The analysis of clinical evidence is fundamental, as in vitro studies, while providing valuable information, may not yield results that can be directly extrapolated for clinical assessment, even when efforts have been made to replicate environments or conditions similar to those of the oral cavity [22, 26]. It is observed that all studies included in this review employed Scotchbond Universal, probably due to its common use in in vitro studies. One study, after 18 months of observation in self-etch mode [1], reported a trend toward better clinical performance. This finding aligns with the recent results of Perdigão et al. [37], who reported a 100% success rate in Class I and Class II restorations one year after placement using Scotchbond Universal in self-etch mode, in a study on permanent teeth. However, a more recent study [21] with a similar observation period reported better performance of restorations in this group when phosphoric acid was applied prior to adhesive application. This was evidenced by a lower percentage of complete or partial loss, improved margin adaptation, and reduced discoloration compared to those that were applied in self-etch mode.

Both studies applied the same USPHS evaluation criteria [38]. This finding may be attributed to the increased etching time, from 15 to 30 seconds, respectively, and to the fact that the most recent study was conducted only on Class II cavities. It is important to highlight that the latter study lacked a detailed description of the caries removal procedure and the adhesion protocol; therefore, its conclusions should be interpreted carefully.

In relation to these findings, a 5-year clinical evaluation [39] of non-carious cervical lesions in permanent teeth concluded that selective enamel etching is fundamental for the survival of this type of restorations. Similarly, a clinical study included in this review, conducted by Ortolani et al. [28], comparing Scotchbond Universal with a single-step self-etch adhesive (Adper Easy One), both applied using the etch-and-rinse technique, concluded that the universal adhesive demonstrated better performance, although it was not significant.

One of the most extensive clinical trial on primary teeth was conducted by Pitchika et al. [18], who followed the clinical performance of self-etch adhesives and etch-and-rinse adhesives for up to seven years. They found that self-etch adhesive restorations were more likely to survive longer. Although the study did not include universal adhesives, it establishes a clinical precedent for the use of simplified adhesive methods. Due to the limited number of available studies, it is important to conduct further clinical trials evaluating the use of universal adhesives, including a larger number of participants [1] and comparing their outcomes with those of two- or three-step adhesive systems. The long-term success of resin restorations begins with an appropriate adhesive procedure; however, it should also be considered that, in line with the needs of pediatric dentists, it is essential to establish a validated simplified protocol that meets the highest quality standards and guides the appropriate selection of restorative materials.

Due to the limited clinical evidence obtained, it is not possible to determine with certainty how universal adhesives perform on primary tooth enamel, especially for the absence of long-term studies and standardized clinical evaluation criteria.

5. Conclusions

Universal adhesives demonstrate effectiveness in vitro performance when compared with self-etch or etch-and-rinse adhesive systems.

The selective etching strategy with phosphoric acid significantly improved the effectiveness of universal adhesives on primary tooth enamel in most in vitro studies.

Due to the lack of clinical study findings, it remains difficult to establish with certainty the clinical effectiveness of universal adhesives on primary tooth enamel.

Availability of data and materials

Not applicable.

Author contributions

DISP—designed the research study; performed the research; wrote the manuscript. DISP, KJCC—analyzed the data. Both authors contributed to editorial changes in the manuscript. Both authors read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Acknowledgment

Not applicable.

Funding

This research received no external funding.

Conflict of interest

The authors declare no conflict of interest.